Audible error detector and controller utilizing channel quality data and iterative synthesis

ABSTRACT

An apparatus and method for detecting and controlling audible errors in a sound communication system at the receiver utilizes channel quality data and also iterative synthesis. Errors occurring in synthesized speech are detected by searching for atypical sound with a stringency dependent upon channel quality. The greater the channel quality deficiency is, the higher the typicality standards will be. Errors are controlled by either re-synthesizing the signal in an iterative way using typicality standards which vary with channel quality deficiency, or by modifying the output signal using typicality standards which vary with channel quality deficiency, or both.

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates generally to the field of audiblecommunication and more particularly to speech coding and channel coding.

BACKGROUND ART

[0002] In a sound communication system, an encoded signal is sent over atransmission channel to a receiver, where the incoming signal is used bya speech decoder to synthesize sound. Channel errors can adverselyaffect synthesized speech provided by the receiver, and there areseveral related art methods for concealing such channel errors.

[0003] The most common method of concealing channel errors is to usecyclic redundancy check (CRC) to detect the errors in the most importantbits and then perform bad frame handling in the speech decoder. Thisusually means replacing the erroneous parameters with the previouslyreceived good ones, or with slightly modified versions of the previousgood ones.

[0004] However, sometimes, this simple method of error detection and badframe handling is not enough to prevent very audible errors fromoccurring. There are basically at least two reasons for this. The firstreason is that the error detection code used in the CRC is often not100% reliable, and therefore some erroneous frames are not marked as badin the channel decoder; these frames, called Undetected Bad Frames(UBF), are then used in the normal speech synthesis in the speechdecoder. This situation can produce very loud and audible artifacts inthe synthesized speech. The second reason for the occurrence of audibleerrors is that, even after erroneous parameters have been replaced bypreviously received error-free parameters, the subsequent bad framehandling is sometimes unsuccessful and may produce audible artifacts.

[0005] One typical related art method for concealing channel errors, inthe situations just described, is to analyze the synthesized speech foratypical speech frames and then conceal those previously undetected orunsuccessfully handled bad speech frames. This type of related artsystem for analyzing and correcting the synthesized speech ofteninvolves concealment units in combination with the channel decoder(including error detection) and bad frame handling, but the concealmentunits can also be implemented standalone (i.e. separately from thechannel decoder and bad frame handling).

[0006] A major problem with related art algorithms for concealing errorsin the synthesized speech is that those algorithms unintentionallyconceal some error-free signals too, thus causing audible artifactsdespite concealment measures. This is because of the rathernon-stationary nature of a speech signal, which makes it ratherdifficult to separate erroneous parts of the synthesized speech. Anotherproblem with the related art is that the concealment methods typicallyinvolve little more than signal attenuation, which merely reduces thevolume of an erroneous sound fragment. A typical related art algorithmis as follows.

[0007] As shown in FIG. 1, an encoded signal on a line 12 is sent over atransmission channel to a receiver 10, where it is provided to a channeldecoder 14 for processing. The channel decoder finds and corrects databit errors, and then provides a channel-decoded signal on a line 16,including a speech parameter signal on a line 18 and a bad frameindicator signal on a line 20. The bad frame indication can be based onany of various different suggestive factors (CRC, channel SNR, signallevel, et cetera). A speech decoder with bad frame replacer 22 isinstructed by the incoming bad frame indicator signal to entirely orpartially replace erroneous parameters with previous good parameters.Subsequently, the speech decoder with bad frame replacer 22 provides asynthesized signal 24 to a signal error analyzer 26 which analyzes thesynthesized signal for erroneous sound characteristics atypical of humanspeech (the typicality standards are not correlated to the quality ofthe transmission channel). If the signal error analyzer does findatypical sound characteristics, it provides a modification commandsignal on a line 30 to a synthesized signal modifier 28 which respondsto the modification command signal by either concealing errors in thesynthesized signal on the line 24, or by allowing the synthesized signalon the line 24 to pass through unchanged if the signal error analyzerdid not detect any significant errors. The synthesized signal modifier'sconcealment of audible errors is accomplished by lowering the signalgain (attenuation), and sometimes by additional actions, resulting in asynthesized output signal on a line 32.

[0008] As mentioned, a problem with related art algorithms is that theysometimes unintentionally detect and conceal error-free signals too,thus causing artifacts that audibly degrade the quality of the outputspeech signal. Thus, the related art involves a tradeoff between twocontrary goals: avoiding changes to any error-free signals, whileensuring changes to all signal errors. Whenever the related art seeks toaccomplish one of these goals, it does so at the expense of reaching theother goal.

[0009] Another problem with the related art algorithms is theineffectiveness of their methods for concealing audible errors. Forexample, if a channel error would have caused a whistling sound, thenthe subsequent corrective modification may still provide an audiblewhistling sound, albeit at a lower and less disturbing volume.

DISCLOSURE OF THE INVENTION

[0010] The objective of the present invention is to improve thedetection and concealment of audio errors occurring as a result ofimperfect transmission channel quality. The invention deals moreeffectively with both of the major problems: audible errors caused byundetected bad frames, and audible errors caused by unsuccessful badframe handling.

[0011] Accordingly, the present invention detects and analyzes atypicalsound with a stringency dependent upon channel quality. The moredeficient the channel quality is, the higher the typicality standardswill be. This use of channel quality data is an advantage over relatedart, which does not correlate typicality standards to channel quality.The present invention deals with perfect channel quality by completelyrelaxing typicality standards, and thus the present invention will notattempt to repair error-free sound as happens in related art. Moreover,because the invention correlates stringency standards to channelquality, this invention becomes more inclined to take corrective actionwhen channel quality is low. This invention thus avoids the verydramatic audible artifacts caused by mistakenly allowing bad framesthrough to the listener.

[0012] The present invention conceals errors by iteratively synthesizingthe incoming signal and/or modifying the signal, in a way that dependsupon channel quality. This is an advantage over related art, in whichsignal synthesis or modification is not correlated to standardsdependent upon channel quality. In this invention, the worse the channelquality, the higher the standards to which the signal is subjected. Inother words, the higher the channel quality deficiency, the higher thetypicality standards. The present invention can be implemented, forexample, at a mobile communication device, or at a base station in awireless communication network, or at both.

[0013] In this invention, an iterative processor is able to providesynthesized signal meeting typicality standards which vary with channelquality deficiency. The iterative processor accomplishes this novelresult by re-synthesizing the signal. However, when only one iterationis used (meaning that re-synthesis does not occur or is absent) thenthis invention still provides for modification of the synthesized signalin order to meet standards varying with channel quality deficiency, andthis is again an advantage over related art wherein error detectionstandards and modification measures do not depend upon channel quality.Thus, according to the present invention, either an iterative synthesisprocess, or a subsequent signal modification process, is correlated tomeet audio typicality standards which vary with channel qualitydeficiency, or alternatively both of these two processes are correlatedto meet audio typicality standards which vary with channel qualitydeficiency. This invention substantially solves the problem ofunintentionally concealing error-free signals, while effectivelyconcealing signal errors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a diagrammatic representation of a typical related artalgorithm for detecting and controlling audible error.

[0015]FIG. 2 is a diagrammatic representation of an embodiment of thepresent invention, for detecting and controlling audible error.

[0016]FIG. 3 is a diagrammatic representation of the iterative processorshowing details not shown in FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] The present invention utilizes iterative synthesis of an encodedsound signal received over a transmission channel. The invention thenprovides an output sound signal having audibly improved accuracy ascompared with conventional methods.

[0018] As shown in FIG. 2, a receiver device 40 receives an encodedsignal on a line 42 received over a transmission channel, and ultimatelyproduces a synthesized output signal on a line 62. This inventionoperates by, first of all, receiving the encoded signal on the line 42over the transmission channel. That incoming signal is processed by achannel decoder 44 which finds and corrects data bit errors, and thenprovides a channel-decoded signal on a line 46 which contains standarddata and may also contain further data including channel qualityinformation. An iterative processor 52 is responsive to thechannel-decoded signal 46 and also may be responsive to measurablechannel quality information received directly from the transmissionchannel.

[0019] The iterative processor 52 provides a synthesized speech signalon a line 54 and may also provide a modification command signal on aline 60 to a synthesized signal modifier 58. The synthesized signalmodifier 58 then modifies sound fragments of the synthesized signal tothe extent instructed by the modification command signal on the line 60,and thus provides the synthesized output signal on the line 62. Thesynthesized output signal 62 may proceed directly from the receiver 40to the user, or may involve further transmission and/or processing(possibly including another iterative synthesis) between the receiver 40and the user shown in FIG. 2. A receiver 40 may, for example, be locatedat a base station in a wireless communication network, and/or located ata mobile communication device, and/or located at other network elementsin a wireless communication network. Thus, a signal from a first mobileuser that finally reaches a second mobile user may have been processedby more than one receiver of the type illustrated in FIG. 2; in thatcase, one iterative processor could be located at the base station whichreceives signals from the first mobile user, and another iterativeprocessor could be located at the second mobile device, so the signalwould be iteratively processed twice during its journey from the firstmobile user to the second mobile user.

[0020] Preferred modification measures performed by the synthesizedsignal modifier 58 include the following measures to conceal atypicalsounds: signal attenuation, spreading the signal spectrum, andattenuating the largest peaks in the synthesized signal spectrum.However, it should be recognized that no modification might be necessarydue to the high performance of the iterative processor 52, combined withhigh channel quality.

[0021] The iterative processor 52 is not limited to synthesizing asingle sound fragment just once, but rather can synthesize the signaliteratively until either a constant upper limit of iterations is reachedor until the signal becomes typical for the type of sound at issue(e.g., speech). In other words, instead of merely replacing bad framesas instructed by the channel-decoded signal 46 and then performing asingle synthesis, the constant number of iterations may be a valuegreater than one so that the iterative processor 52 also replaces badframes identified as atypical by the iterative processor 52 itself. Ineach iteration, replacement of bad frames varies, so that each iterationwill produce a slightly different synthesized speech signal. Thereplacing of bad frames can be varied between iterations by, forexample, changing the attenuation factor of energy parameters, or bymanipulating spectral parameters in order to flatten or sharpen thespectral peaks in the signal, or by using some of the parameters from abad frame based upon an iteration-dependent comparison between thecurrent number of parameters and the number of parameters from theoriginal iteration.

[0022] The iteration is stopped when the speech signal apparentlyfulfills the typicality standards currently active, or when theiteration limit is reached. Only if the constant upper limit ofiterations is reached will the synthesized signal modifier 58 perform anon-zero modification of the signal, as opposed to simply allowing thesignal to pass through without modification by the synthesized signalmodifier 58.

[0023] Each time a sound fragment is analyzed for typicality inside theiterative processor, the accuracy of that analysis is enhanced by theuse of channel quality data. The channel quality data is vital forreliably responding to sound level errors, according to the presentinvention.

[0024] Channel quality information may enter the iterative processor 52included in the channel-decoded signal on the line 46, for example aspseudo bit error rate (BER) calculated inside the channel decoder 44.Measurable channel quality information may also enter the invention'siterative processor 52 directly as non-decoded channel qualityinformation contained in the encoded signal on the line 42 received overthe transmission channel. The quality of the used transmission channelcan be estimated by various different ways, in addition to BER (e.g.frame error rate, pseudo BER, signal to noise ratio, et cetera).

[0025] If there is no channel quality deficiency, then the errordetection process is disabled, meaning that the iterative processor 52relaxes its standards for typical sound to the point of nonexistence.This ensures, among other things, that the invention does not mistakenlyalter synthesized signals that are actually error-free.

[0026] This invention is substantially based upon the principle that asingle erroneous frame mistaken as a good frame can cause more damage tooutput speech quality than many mistakes in which a good frame ismisinterpreted as a bad frame. Therefore, the more erroneous theincoming data (i.e., the lower the quality), the more inclined thereceiver should be to interpret each single frame as erroneous. This isaccomplished by the present invention, in which the iterative processorapplies greater stringency for sound typicality when channel qualitydata shows lower channel quality.

[0027] As shown in FIG. 3, this invention includes a mechanism withinthe iterative processor 52 for accomplishing the purposes justdescribed. Within the iterative processor 52, a speech decoder with badframe replacer 70 responds to the channel decoded signal on the line 46(which includes a speech parameter signal on the line 78), by replacingbad parameters with previously received good parameters. Then the speechdecoder with bad frame replacer 70 synthesizes the signal, and providesthis synthesized speech signal on a line 54 to the signal error analyzer90 which examines the synthesized speech signal to see if there arefragments atypical for speech.

[0028] This examination for non-typicality, which occurs within thesignal error analyzer, is correlated to the channel quality, with morestringent typicality standards corresponding to lower quality ofservice. As discussed above, the channel quality information ismeasurable from channel quality information contained in the encodedsignal on the line 42 received over a transmission channel, and/orchannel quality information is provided by the channel-decoded signal onthe line 46 (e.g. pseudo BER). It is also possible for the signal erroranalyzer to be influenced by bad frame indication information andreceived speech-coding parameters (discussed below). In any case, thesignal error analyzer 90 applies the typicality standards to thesynthesized speech signal, and this can be done in several ways,including, for example, analyzing absolute and relative energy-levelchanges between successive speech frames.

[0029] According to a first embodiment of the present invention, if thesignal error analyzer 90 finds atypical sound or speech fragments, thenthe speech decoder with bad frame replacer 70 further replaces badframes with previously received good parameters, and re-synthesizes thesignal. This iterative process repeats until the sound becomes typicalor until an upper limit (N) of iterations is reached. Then the signalerror analyzer 90 may provide a modification command signal on the line60 which, as explained previously, is used outside the iterativeprocessor 52 by the synthesized signal modifier 58 to determine whetherand how to modify the synthesized speech signal on the line 54.

[0030] In a second embodiment of the present invention, the upper limitof iterations N is simply the number one (N=1), and re-synthesis doesnot occur or is absent. However, this is still a very differentsituation from the related art, because the signal error analyzer 90sends a modification command signal on the line 60 to the synthesizedsignal modifier 58 requiring that the signal be modified depending uponthe channel quality, wherein typicality standards vary with channelquality deficiency. In contrast signal modification in the related artdoes not use channel quality information in this way. Note that thefirst embodiment of the present invention, discussed above, may alsoemploy signal modification which uses channel quality information in thesame manner as the second embodiment, in which case the differencebetween the two embodiments is that re-synthesis cannot occur in thesecond embodiment.

[0031] In the first embodiment of this invention, the speech decoderwith bad frame replacer 70 will replace frames that are determined to beatypical for speech by the signal error analyzer 26, instead of onlyreplacing bad frames identified by the channel decoder 44, and thisinvention includes a detailed process for achieving this. Whenever a newset of parameters arrives via the speech parameter signal on the line78, the speech decoder with bad frame replacer 70 immediately provides astate signal on a line 104 to a decoder storage 102 enabling the decoderstorage 102 to preserve the internal states of the speech decoder withbad frame replacer 70 for use in succeeding iterations.

[0032] In the first iteration, the speech decoder with bad framereplacer 70 only uses parameters from the speech parameter signal on theline 78, if no bad frame is indicated by the channel decoder. If a badframe is detected by the channel decoder, then, in the first iteration,parameters from the decoder storage 102 are usually used, oralternatively part of the parameters are taken from the line 78 and partare taken from the storage 102. In the following iterations, the speechdecoder with bad frame replacer 70 uses parameters from the decoderstorage 102, regardless of whether a bad frame was indicated by thechannel decoder.

[0033] Before the speech decoder with bad frame replacer 70 begins eachsynthesis or re-synthesis within an iterative cycle, it receives a badframe substitution signal on a line 88 instructing it whether or not toreplace frames prior to synthesis. If replacement is not required, thenthe substitution signal on the line 88 indicates zero replacement; butif a replacement is required, then the substitution signal on the line88 will indicate one replacement. This bad frame substitution signal onthe line 88 comes from the bad frame counter 82, and the nature of thissignal is determined by whether the bad frame counter 82 has a value ofzero or a value of one.

[0034] The bad frame counter 82 is reset to zero immediately after eachsynthesis iteration in response to a reset signal on a line 84. The badframe counter 82 will be put into value one if the channel decoder 44indicated a bad frame and no synthesis has yet occurred. The bad framecounter 82 will also be put into value one if the channel decoder 44indicated a good frame but the signal error analyzer 90 detected anatypical frame. Both of these situations produce the same result, inthat they both force the bad frame counter 82 from value zero to valueone. This count from zero to one happens in response to a count signalon a line 86 from a logical port 94. The logical port 94 performs alogical OR operation in response to two factors: whether the signalerror analyzer 90 encountered an atypical frame and whether the channeldecoder 44 detected a bad frame, and these factors are represented by acharacteristics error signal on a line 92 from the signal error analyzer90 and a bad frame indicator signal on the line 96 contained in thechannel decoded signal on the line 46.

[0035] As mentioned previously, channel quality information may enterthe iterative processor 52 included in the channel-decoded signal on theline 46, for example as pseudo BER, and may also enter the invention'siterative processor directly as channel quality information included inthe encoded signal received over a transmission channel on the line 42.In both of these cases, the channel information would be input into thesignal error analyzer 90 which interprets the channel qualityinformation so as to establish stringency standards for typical speech(higher standards for lower channel quality).

[0036] The synthesized speech signal on the line 54 produced by theiterative processor 52 may enter the synthesized signal modifier 58directly from the speech decoder with bad frame replacer 70, in whichcase the modification command signal on the line 60 instructs thesynthesized signal modifier 58 to discard synthesized signals that arestill in the process of iterative re-synthesis. However, this inventionalso includes a simple switch 98 to ensure that only a synthesizedspeech signal 54 that has been completely re-synthesized will enter thesynthesized signal modifier 58. The operation of the switch is governedby a switch control signal on a line 100 from the signal error analyzer90, so that the switch selects not to allow any signal to pass throughunless the switch control signal indicates that the iteration iscomplete. Thus, due to the switch, the synthesized signal modifier 58will usually pass the received signal through without taking any actionsto modify it, because the iteration process will have usually produced asound signal within typical range.

[0037] The present invention has numerous possible applications, and canbe used at the receiver end of a digital wireless telecommunicationssystem, in particular a wireless telephone system. In this application,the standards applied to sound signals will be commensurate with humanspeech.

[0038] It should be recognized that each signal described in thisdisclosure is defined broadly as a cause and effect relationship. Thesignal may be direct or indirect, may comprise any number ofintermediate steps, and may be integrated together with other signals,as will be understood by those skilled in the art.

[0039] Although this invention has been shown and described with respectto a best mode embodiment thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions and additions in the form and detail thereof may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. An apparatus comprising: a channel decoder (44),responsive to an encoded signal received over a transmission channel(42), for providing a channel decoded signal (46); and an iterativeprocessor (52), responsive to the channel decoded signal (46), forproviding a synthesized speech signal (54) meeting typicality standardswhich vary with channel quality deficiency.
 2. The apparatus defined inclaim 1, wherein the iterative processor (52) comprises: a speechdecoder with bad frame replacer (70), responsive to the channel-decodedsignal (46), for providing the synthesized speech signal (54); and asignal error analyzer (90), responsive to the synthesized speech signal(54) and responsive to the channel decoded signal (46), for providing acharacteristics error signal (92) to which the speech decoder with badframe replacer (70) is responsive, wherein the signal error analyzer(90) applies typicality standards which vary with channel qualitydeficiency.
 3. The apparatus defined in claim 1, wherein the iterativeprocessor comprises: a speech decoder with bad frame replacer (70),responsive to a speech parameter signal (78) included in thechannel-decoded signal (46), and responsive to a bad frame substitutionsignal (88), for providing the synthesized speech signal (54); a badframe counter (82), responsive to a reset signal (84) and responsive toa count signal (86), for providing the bad frame substitution signal(88); a signal error analyzer (90), responsive to the channel-decodedsignal (46) and responsive to the synthesized speech signal (54), forproviding the reset signal, and for providing a characteristics errorsignal (92); and a logical port (94), responsive to the characteristicserror signal (92) and also responsive to a bad frame indicator signal(96) included in the channel decoded signal (46), for providing thecount signal (86).
 4. The apparatus defined in claim 3, wherein theiterative processor (52) further comprises a switch (98), responsive toa switch control signal (100) from the signal error analyzer (90) andalso responsive to the synthesized speech signal (54), for selectivelyproviding the synthesized speech signal (54).
 5. The apparatus definedin claim 3, wherein the iterative processor (52) further comprises adecoder storage (102), responsive to a state signal (104) from thespeech decoder with bad frame replacer (70), for providing a statesignal (104) back to the speech decoder with bad frame replacer (70). 6.The apparatus defined in claim 1, wherein the iterative processor (52)is also directly responsive to the encoded signal received over atransmission channel (42).
 7. The apparatus defined in claim 1, whereinthe apparatus is a mobile communication device.
 8. The apparatus definedin claim 1, wherein the apparatus is a network element in a wirelesscommunication network.
 9. The apparatus defined in claim 8, wherein thenetwork element is a base station.
 10. An apparatus comprising: achannel decoder (44), responsive to an encoded signal received over atransmission channel (42), for providing a channel-decoded signal (46);an iterative processor (52), responsive to the channel decoded signal(46), for providing a synthesized speech signal (54) and for providing amodification command signal (60); and a synthesized signal modifier(58), responsive to the synthesized speech signal (54) and to themodification command signal (60), for providing a synthesized outputsignal (62) meeting typicality standards which vary with channel qualitydeficiency.
 11. The apparatus defined in claim 10, wherein the apparatusis a mobile communication device.
 12. The apparatus defined in claim 10,wherein the apparatus is a network element in a wireless communicationnetwork.
 13. The apparatus defined in claim 12, wherein the networkelement is a base station.
 14. The apparatus defined in claim 10,wherein the iterative processor (52) performs only one iteration,without re-synthesis.
 15. The apparatus defined in claim 10, wherein theiterative processor (52) comprises: a speech decoder with bad framereplacer (70), responsive to the channel-decoded signal (46), forproviding the synthesized speech signal (54); and a signal erroranalyzer (90), responsive to the synthesized speech signal (54) andresponsive to the channel-decoded signal (46), for providing themodification command signal (60).
 16. The apparatus defined in claim 15,wherein the iterative processor (52) performs only one iteration,without re-synthesis.
 17. The apparatus defined in claim 10, wherein theiterative processor (52) comprises: a speech decoder with bad framereplacer (70), responsive to a speech parameter signal (78) included inthe channel-decoded signal (46), and responsive to a bad framesubstitution signal (26), for providing the synthesized speech signal(54); a bad frame counter (82), responsive to a reset signal (84) andresponsive to a count signal (86), for providing the bad framesubstitution signal (88); a signal error analyzer (90), responsive tothe channel-decoded signal (46) and responsive to the synthesized speechsignal (54), for providing the modification command signal (60), forproviding the reset signal (84), and for providing a characteristicserror signal (92); and a logical port (94), responsive to thecharacteristics error signal (92) and also responsive to a bad frameindicator signal (96) included in the channel decoded signal (46), forproviding the count signal (86).
 18. The apparatus defined in claim 17,wherein the iterative processor (52) further comprises a switch (98),responsive to a switch control signal (100) from the signal erroranalyzer (90) and also responsive to the synthesized speech signal (54),for selectively providing the synthesized speech signal (54).
 19. Theapparatus defined in claim 17, wherein the iterative processor (52)further comprises a decoder storage (102), responsive to a state signal(104) from the speech decoder with bad frame replacer (70), forproviding a state signal (104) back to the speech decoder with bad framereplacer (70).
 20. The apparatus defined in claim 17, wherein theiterative processor (52) is also directly responsive to the encodedsignal received over a transmission channel (42).
 21. A methodcomprising the steps of: providing a channel-decoded signal (46) inresponse to an encoded signal received over a transmission channel (42);and executing an iterative signal processing step, in response to thechannel-decoded signal (46), for providing a synthesized speech signal(54) meeting typicality requirements which vary with channel qualitydeficiency.
 22. The method defined in claim 21, wherein the iterativesignal processing step comprises the steps of: providing the synthesizedspeech signal (54) in response to the channel-decoded signal (46); andproviding a characteristics error signal (92) responsive to thesynthesized speech signal (54) and responsive to the channel-decodedsignal (46).
 23. The method defined in claim 21, wherein the iterativeprocessing step is also executed in direct response to the encodedsignal received over a transmission channel (42).
 24. A methodcomprising the steps of: providing a channel-decoded signal (46) inresponse to an encoded signal received over a transmission channel (42);executing an iterative signal processing step, in response to thechannel-decoded signal (46), for providing a synthesized speech signal(54) and for providing a modification command signal (60); and providinga synthesized output signal (62) meeting typicality standards which varywith channel quality deficiency, in response to the synthesized speechsignal (54) and also in response to the modification command signal(60).
 25. The method defined in claim 24, wherein the iterative signalprocessing step is executed only once, without re-synthesis.
 26. Themethod defined in claim 24, wherein the iterative signal processing stepcomprises the steps of: providing the synthesized speech signal (54) inresponse to the channel-decoded signal (46); and providing themodification command signal (60) in response to the synthesized speechsignal (54) and also in response to the channel decoded signal (46). 27.The method defined in claim 26, wherein the iterative signal processingstep is executed only once, without re-synthesis.
 28. The method definedin claim 24, wherein the iterative processing step is also executed indirect response to the encoded signal received over a transmissionchannel (42).